Flame pocket fluid fuel burner



u y 1950 H. K. J. VAN DEN BUSSCHE 2,515,845

- FLAME POCKET mun 1mm. BURNER Filed lay 13, 1947 2 Sheets-Sheet 1\nveni'or:

H. K. J. VAN DEN BUSSCHE W FLARE POCKET FLUID FUEL BURNER 2 Sheets-Sheet2 Eiled lay 13. 1947 I H A r/ 4.0 15-0 L in Inches Fig 4 m w $52.: 5 O

Patented July 18, 1950 FLAME rooxnr FLUID FUEL BURNER Hendrik K. J. vanden Bussche, Delft, Netherlands, assignor to Shell Development Company,San Francisco, Calii'., a corporation of Delaware Application May 13,1947, Serial No. 747,607 In the Netherlands June 25,1946

Claims.

This invention relates to an improved burner for burning inflammablegaseous mixtures issuing in stratic form from a slit or series of slits,and is particularly concerned with attaining steady and efficientcombustion at high flow velocities. The inflammable gaseous mixture maybe a mixture of air (or other oxidizing gas) with a gaseous fuel, e. g.,butane gas, or finely-divided liquid fuel, e. g., atomized or sprayeddroplets of oil, or finely divided solid fuel, e. g., powdered coal, itmay, therefore, be a gas or a dispersion of a liquid or of a solid in agas.

The invention may be applied in any situation where such inflammablegaseous mixtures are burned, such as boilers and other types offurnaces. It is particularly useful for aircraft gas turbines, eitherfor plain jet propulsion or for combined airscrew and. jet propulsion,wherein it is necessary that combustion occurs within a small space athigh flow velocities of the inflammable fluid, and that the flame sticksto the burnerunder all circumstances.

Fuel and combustion air are customarily mixed in the combustion or flamechamber, usuall by atomization and/or vaporization, aided in some casesby turbulence. The use of fuel-air mixtures which have beencarburettedbefore entering the combustion chamber offer certainadvantages, but such mixtures will not always stick to the burner, i.e., the flame is subject to leaving the combustion chamber at high flowvelocities, resulting in the flames being extinguished. Moreover, it hasbeen diflicultto attain complete combustion within a limited spaceforhigh rates of flow.

It is an object of this invention to provide an improved burnerforburning gaseous inflammable mixtures within a confined flame chamber.A further object is to provide a flame which is provided withstabilizing whirls on both sides of a stratic current of the inflammablemixture in the flame chamber, whereby the more complete and rapidcombustion of the inflammable mixture can be attained even at high flowvelocities.

According to the present invention the gaseous inflammable mixture ismade to flow into a flame chamber in stratic form, i e., in the form ofa sheet or thin wall, by issue through a slit at or near the rear of theflame. chamber, and the flame is formed so that a vortex of flame gases,i. e., hot burning or burned gases, is maintained on either side of thestream of the gaseous mixture issuing from the slit. These vortices,which may take the form of closed, annular vortices, convey the hotgases back to the point where the inflammable mixture issues into theflame chamber, thereby rapidly heating the issuing mixture and ignitingit. The result of this is that 'very high velocities through the slitcan be employed without blowing out the flame. With the flame chamberdescribed herein the rotational velocity of the vortices increases withthe velocity at which the inflammable gaseous mixture is supplied, sothat the flame is self-stabilizing.

A flame of the type contemplated by the present invention can be insuredonly in flame chambers of certain shapes, as will be explainedhereafter, while with other shapes the desired flames either are notattainable, or will be realized only sporadicalhr. The required shapefor the flame chamber is dependent, among other things, upon thecomposition of the gaseous mixture, the shape of the slit, and thedirection in which the mixture issues from the slit. However, once thedesirability of using a flame of the type described in thisspecification and its characteristics are known,

the required shape can be ascertained experimentally by those skilled inthe art, although it is also possible to state theoretically therequirements to which the flame chamber must conform. The shape must besuch that the backward flow of air (1. e., toward the slit) between thewall of the flame chamber and the flame is prevented. Hence, the frontof the flame chamber, 11. e., the end where the flame leaves thechamber, must closely conform to the desired shape of the flame.

If the slit is annular the flame chamber should have its inner wallsshaped as a surface of revolution, e. g., like a circular truncated coneor circular cylinder, of sufiicient length to enclose the flame at leastpartially. This is true also when the slits do not form continuousannuli, aswhen they are formed of a series of apertures close togetherbut arranged on the circumference of a circle, and when severalconcentric annuli of continuous slits or series of apertures areprovided. But when other forms of slits are used, the flame chamber mayhave other cross-sectional outlines, conforming to the stratic stream ofthe fluid mixture, which may, for example, be boxlike, composed of fouror more straight sides.

The invention will be explained in greater detail by reference to thedrawings forming a part of this specification, wherein:

Figs. 1, 2 and 3 are longitudinal cross-sectional views of flamechambers showing three types of flames, of which the flame shown in Fig.3 is the one sought in the present invention;

Fig. 4 is a graph showing the types of flames acme obtainable with onekind of flame chamber when the length and diameter are varied.

In Figs. 1 to 3, l is the wall of a flame chamber, the inner surface ofwhich is a surface of revolution, flared slightly toward the mouth. Thechamber has an opening in its rear wall for receiving a supply conduit5, having an annular slit 6, through which an inflammable gaseousmixture issues into the flame chamber. The annular slit may be formed bymounting a circular core 1 in the conduit 5. The wall of the flamechamber 4 may be made of refractory material, such as fire brick, andmay be part of the combustion chamber lining which is recessed to formthe flame chamber; it may, however, also be a separate cup-shaped piecesecured to the supply conduit 5, and constructed of refractory materialor of metal, or metal lined with refractory material on its inner face.

It was found that, with burners of the type illustrated, three generalflame forms may be formed, these forms being illustrated in Figs. 1, 2and 3, and herein designated as forms I, II, and III, respectively.Flame form I results when no flame chamber is provided or when a flamechamber having a short length in relation to its diameter is used. Inthis shape of flame the burning gases advance forwardly like a slightlydiverging tube. Within the tube an annular vortex 8 of burned andburning gases is formed, while external gas or air, from points outsideof the flame chamber, flows in as shown by the arrows 9. Both phenomenaare the result of the entraining effect of the gaseous mixture as itissues from the slit, which lowers the pressure, i. e., produces apartial vacuum. Measurements have indicated that a moderate vacuum isformed inside of the flame at the rear of the flame chamber (at the faceof the core 'I) herein referred to as the core vacuum P0. The vacuum atthe side wall of the refractory cone, or flame chamber cup near its rearwall, herein referred to as the cone vacuum Pk is very low, beingdissipated by the inrushing air. In this form of flame the inrushingoutside air cools the outside of the flame, and the mixture burnssubstantially only on the inside and combustion is not complete.

Flame form II, shown in Fig. 2, may be formed with a longer flamechamber. In this form the flame spreads laterally and no air can enterbetween the flame and the wall of the chamber. A moderately high conevacuum Pk results, causing the formation of annular vortices l ofburning and burned gases. There is also a high core vacuum Pc, but theformation of appreciable central vortices is prevented by air enteringcentrally as shown by the arrows ll. Rudimentary vortices l2 may occur,but these contain largely gases which are still unburned. In this formof flame the mixtures within the flame chamber burns substantially onlyon the outside and a considerable part of the fuel is not burned.

4 a result, it is not blown oil the slit even with very great velocitiesof flow through the slit. Moreover, with this shape of flame completeburning is achieved, either without or with a very slight excess of air.It follows that the shape of flame shown in Fig. 3 oflers greatadvantages.

The Flame form III may, also, be deflned with reference to the core andcone vacua, which, as explained above, are lower and higher,respectively than with the other flame forms. The desired flameaccording to this invention may further be distinguished from the otherflame forms by the higher flame pressure, viz., the difference betweenthe pressure in the conduit 6 when the gaseous mixture is not ignitedand the pressure during burning. This pressure is always lower withoutignition, and becomes progressively higher with Flame forms II, I, andIII, respectively.

Which type of flame will prevail will depend upon several factors, aspreviously mentioned. Of these, the dimensions of the flame chamber areof greatest importance, but the proper dimensions depend upon the mannerin which the inflammable mixture issues from the slit. The effect ofthese dimensions will be illustrated in connection with a burner inwhich a mixture of air and butane in about theoretically equivalentamounts for complete combustion was passed through an annular slit intovarious refractory flame chambers. The slit had an inside diameter of 26mm. and an outside diameter of 28 mm., 1. e.

the slit was 1 mm. wide, and the butane-air mixture was given atangential velocity by flowing it through a whirl chamber beforereaching the slit to cause the ratio of the axial velocity to tangentialvelocity to be 4:1. The flame chambers were in all cases shaped about asshown in Figs. 1 to 3, with a flare of 7 (each wall sloped 3 from theaxis). Diameters are measured at the base or narrowest part of the sidewalls, and the lengths from the base of the side wall to the orifice,parallel to the axis. The rear wall, behind the base,

was inclined 80 to the axis. r The graph of Fig. 4 shows the effect ofusini; variously dimensioned flame chambers, the

Finally, Fig. 3 shows Flame form III, the shape of flame which is soughtaccording to this invention. In this case the flame chamber is longerthan in the preceding flgures, and the gaseous mixture, after issuingfrom the slit as a tubular sheet, first opens and later more or lesscloses, and annular vortices are formed, both inside of the tube, asshown at l3, and outside of the tube, as shown at it. With this shape offlame the core vacuum Fe is low an. the cone vacuum Pk is high. Thegases burr. both on the inside and outside of the tube of gases and theflame is stabilized both on the inside and on the outside; as

lengths L being slotted as abscissae and the diameters as ordinates allin inches withflame chambers having dimensions which would be plotted inthe upper, left-hand area, marked I Flame form I (having exclusively aninside vor-.

tex) is obtained; and with dimensions falling in the lower, right-handarea, marked lII, Flame form III (having both inside and outsidevortices) is obtained. Flame form II can occur with dimensionscorresponding to the areas marked 11 which, as can be seen, formtransition areas between areas I and III. In these transition areasaerodynamic labile conditions prevail, so that, as is shown on thegraph, with certain values of L and D, two and sometimes even all threeflame forms can exist, which, due to some external cause or other, cansuddenly change from one shape to another.

"It is clear from the graph of Fig. 4 that, in order to obtain thedesired Flame-form III, the flame chamber must-for a given diameter-haveat least a certain maximum length, while, on the other hand, for a givenlength the diameter may not exceed a certain maximum value. For theparticular feed composition, shape of slit, etc., of these experiments,Flame form III is usually insured when the flame chamber is at least aslong as required by the inequality:

L 1.2D-.55 inch (1) atomizing cup surrounded a Flame form I issporadically attained for lower lengths, down to about 1.2D1.50 inches.

For other conditions the locations and slopes of the lines shown on Fig.4 will differ, but these lines will have a generally similar relation.For example, in tests with the same series of refractory flame chambers,fuel-air composition, slit and flow rate, in which the tangentialvelocity was reduced, form I was not so readily attained, and somewhatlonger chamber became necessary; Flame form I could be insured only bymaking:

L 1.4D.83 inch (2) and was sporadically attained for lengths down to1.4D1.42 inches.

When the slit was increased while maintaining the same tangentialvelocity as in the original tests (whereby the ratio of axial totangential velocity was decreased) the desired flame form was stabilizedand shorter lengths could be used. In this case, with a slit 2 mm. wideFlame form I could be insured when:

L 0.9lD-.51 inch (3) while Flame for I could be realized for lengths aslow as 0.9lD-l.94 inches. With a slit 3 mm. wide. the requirement wasmason-"0.95 inch (4) to insure Flame form I, while this form could beattained occasionally with even shorter lengths.

It was found that the rotational motion of the gaseous mixture exerts afavorable stabilizing action on the desired flame form and I prefer, forthat reason, to impart to the advancing air a tangential component,although the invention may be practiced even without this feature. Whenthe tube-like stratum issuing into the flame chamber has atangentialcomponent the vortices are deviated from closed loops by the tangentialvelocity; in this case the flow lines of gas particles in each eddy maybe regarded as helical lines on a torus.

With flame chambers having dimensions which may be plotted in thediagonal strips of Fig. 4 marked with several Roman numerals, the flameshifts from one form to another, and no generally valid rule forpredicting the form can be given. In certain experiments, however, itwas found that frequently the flame starts with Flame form 1; increasingthe fuel-air ratio causes a change from Flame form I into Flame formIII, which then remains after the ratio was again reduced. Reducing theratio to well below the theoretical ratio for complete combustionresults in a change into Flame form II. At higher pressures of thegaseous mixture in the feed conduit Flame form I could be made to changeinto Flame form HI either by increasing or decreasing the fuel-airratio.

As was suggested previously, it is possible to bum'various kinds ofgaseous fuel mixtures in this burner, including those containinggaseous, liquid and solid fuels. As a specific example for burningliquid fuel, the type having a rotating by an annular air .sllt may becited; in this case the li uid oil is thrown from the edge of the cup ine droplets into an enveloping current of air, which may have atangential component, resultingiin an inflammable mixture of liquid oilin air. Atomiz ing burners of this general type are known U. S. PatentsNos. 2,184,471 and 2,214,568) a need not be described further herein.When such a rotating atomizing cup burner is applied, the conduit 5shown in the drawing of this specification is the outer air duct and thecore I is replaced by the rotating cup.

It is, moreover, not essential to have an annular slit for theapplication of the invention; for example, flat slits parallel to oneanother can also be used in which case, of course, care must be takenthat the vortices are not hindered in their existence or continuance bythe shape of the flame chamber. The vortices must either be closed inthemselves or be next to a wall of the flame chamber.

I claim as my invention:

1. A burner comprising a flame chamber having a rear wall and a closedside wall and open at the front end, the length of said side wall ininches exceeding a distance equal to 0.80D0.95, wherein D is theinternal diameter of the side wall near the rear wall, said length beingat least 1.5 inches and the front diameter of said chamber being limitedrelatively to said diameter near the rear wall to provide asubstantially tubiform flame chamber, and an annular slit in said rearwall having a diameter not more than about 0.73D and disposed forintroducing an inflammable mixture forwardly in the form of a tubularstratum spaced from said side wall, whereby a flame formed by ignitingsaid inflammable stratic mixture will form vortices of flame gases bothwithin the tubular stratum and between the outside of the tubularstratum and said side wall for rapidly heating themixture issuing fromthe slit and thereby stabilizing it against extinction at high rates offlow.

2. The burner according to claim 1 in which the slit has a circularshape and the side wall of the flame chamber has a surface ofrevolution.

3. The burner according to claim 1 wherein the flame chamber has theshape of a slightly diverging irustrum of a cone and is fully open atthe front end thereof.

4. The burner according to claim 1 wherein the diameter of the annularslit is not less than about 0. 8D.

5. The burner according to claim 1 wherein the length of the side wallexceeds a distance equal to 1.4D0.83D.

HENDRIKK. J. van Dxx BUSSCHE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

